The proposed research plan is centered upon the quantification of left atrial-left ventricular interactions. The hypothesis to be tested is that current techniques of myocardial preservation, as applied to patients with current indications for cardiac surgery, do not afford adequate protection of the left atrium. With the broadening indications for cardiac surgery, patients with impaired ventricular function will require the preservation of atrial and mitral valve function for survival. Additionally, impaired ventricular function, both mechanical and electrophysiologic, can lead to abnormalities of mitral valve function that will also adversely effect global cardiac function. Arrhythmias, both ventricular and supraventricular are common following cardiac operations, and are due to the same deficits in preservation that lead to atrial dysfunction. As well, chronic valvular heart disease leads to chronic supraventricular arrhythmias, despite valve replacement or repair. Left atrial isolation, a method to be used in the study of atrioventricular interaction, is also a potential electrophysiologically specific operation to minimize the deleterious effects of supraventricular arrhythmias on cardiac function in patients with intrinsic mitral valve disease. Atrioventricular interactions will be studied in an animal model using echocardiography as a screening procedure. The model will incorporate features of cardioplegic arrest, coronary occlusion, programmed electrical stimulation and left atrial isolation to mimic changes in cardiac surgery and to isolate functional aspects of atrioventricular interaction. Detailed analysis of left atrial function will be accomplished using ultrasonic dimension transducers, high fidelity intra-cavitary pressure measurement and the electromagnetic measurement of transmitral blood flow. The above-mentioned techniques will be used to perturb the model systems. Using computer analysis, the left atrium will be modeled as a hemispheroid and left atrial work quantified. Left atrial isolation will also be studied in its own right in a colony of animals subjected to this procedure followed by prolonged survival.